1. Field of the Invention
The present invention relates to an apparatus for generating a pulse width modulation signal having, for example, a pulse width corresponding to an image gradation degree.
The present invention relates to a triangular wave signal generating apparatus used with a pulse width modulation signal generator or the like.
The present invention relates to a pixel modulation circuit capable of modulating a pixel in response to clocks having a period shorter than pixel clocks.
2. Related Background Art
A PWM pixel modulation method for a laser beam printer (LBP) is known in the art whereby a laser beam radiation time is controlled for each pixel to obtain a light amount correlated to the printing density (deposited toner amount) suitable for a highly fine (high gradation) video image.
FIG. 1 illustrates such pixel modulation. A video clock (FIG. 2(a)) representing a pixel unit and synchronizing with a beam detect (BD) pulse indicating a horizontal reference position of a printing sheet, is inputted to an input terminal 31. The video clock signal is converted into a triangular wave (FIG. 2(d)) synchronously with the video clock signal, and the triangular wave signal is supplied to a comparator 41. Pixel data (FIGS. 2(b)) of eight bits for example is inputted to an input terminal 45, the pixel data being used for determining the printing density of each pixel. The input pixel data is latched (FIG. 2(c)) by a latch 46 in response to the video clock. An output of the latch 46 is converted into an analog voltage (FIG. 2(d)) by a D/A converter 35, the analog voltage being supplied to a comparator 41. As shown in FIG. 2(d), the comparator 41 compares the input triangular wave signal with the pixel analog voltage to output a laser drive pulse (FIG. 2(e)) pulse-width-modulated in accordance with the density of pixel data.
A laser beam is radiated, for example, while the laser drive pulse takes an H level. Therefore, pixel data DN+2 corresponds to a "deep pixel", and pixel data DN corresponds to a "light shaded pixel". The printing density is very sensitive to a pulse width (radiation time). For a high image quality, it is therefore necessary not only to be able to change the peak level value and d.c. offset value of a triangular wave in accordance with the environmental conditions, but also to make the triangular wave stable.
In the triangular wave generator circuit 43 shown in FIG. 1, the video clock is shaped by a buffer 56 to eliminate noises such as ringing, and transformed into a triangular wave by a time constant circuit having a time constant T=R31 * C13 larger than the clock period.
The level of the triangular wave can be set by the resistance value of R31, and the d.c. offset can be set by VR1 with a sufficiently large capacitance of C14. In order to ensure the linearity of the triangular wave slope, it is necessary to set the time constant T about three times the video clock period.
The triangular wave generator circuit shown in FIG. 1 has the following demerits.
1) Since the duty of an input clock signal is not definite, it is necessary to generate clock signals having a period at least two times as high as that of original video clocks. This necessitates an expensive crystal oscillator operating at a higher frequency, for an LBP system expected to operate inherently at high speed for the highly fine printing.
2) The level of a triangular wave signal is required to be at least about 0.6 to 0.7 Vpp in order to match the output characteristic of a general high speed D/A converter. It is therefore necessary to drive the buffer 56 at a large level of about 12 V, which is a disadvantage in configuring an IC.
3) The time constant T for determining the level of a triangular wave changes with an environmental condition. In the pixel modulation for an LBP, the temperature of the circuit is generally monitored, imposing a large load in configuring a PWM pixel modulation system for an LBP.
4) The PWM characteristic is determined by a combination of the level and offset of a triangular wave, making the adjustments of both the level and offset difficult.
A pixel modulation method is also known wherein a pixel is modulated by pixel data in unit of the pixel clock divided by N.
For example, in a digital copier or LBP, an image is represented in units of pixel clocks. FIGS. 3(A) to 3(C) show a character "A" represented by digital signals. FIG. 3(A) shows an ideal character "A", and FIG. 3(B) shows the character "A" represented in units of pixel clocks. As seen from FIG. 3(B), the character "A" has a low resolution and a poor linearity of a sloped line. The improved image quality of the character "A" is shown in FIG. 3(C) in which the pixels are modulated in units of pixel clocks used with FIG. 3(B) divided by four.
In determining whether each pixel divided by four is black or white, the data of the character "A" shown in FIG. 3(B) is referred to. Namely, if the data of FIG. 3(B) is determined as representing an oblique line, each pixel divided by four is determined as black or white so as to smooth the step of the oblique line.
The modulation operation for the character shown in FIG. 3(C) will be described with reference to the timing chart of FIG. 5. In FIG. 5, CK1 is the same image clock used with the character shown in FIG. 3(B). CK2 represents a clock signal having a frequency four times that of CK1. A to D represent modulation data for modulating each pixel divided by four.
Data A to D represent white or black data of each pixel divided by four, as shown in FIG. 3(C).
For the pixel modulation, four bit parallel data A to D is converted into serial data at the timing of CK2. This parallel/serial conversion is performed by a shift register. FIG. 4 shows a shift register circuit, and FIG. 5 is a timing chart explaining the operation thereof. With the clocks having a frequency four times as high as that of the clocks used for the character shown in FIG. 3(B), and with the four bit modulation data, the character image shown in FIG. 3(C) can be obtained.
The frequency of clocks used for the character shown in FIG. 3(B) becomes higher and higher as the printing speed and image fineness of a digital copier or LBP are improved. The clock frequency used with FIG. 3(C) is four times as high as that used with FIG. 3(B). When the modulation clock frequency becomes in excess of 80 to 100 MHz, there arises a CMOS process limit, i.e., expensive ECL logics and crystal oscillator, leading to a high cost of the apparatus.